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When NASA announced its probe to study the Sun from record close distances one of its stated goals was to sample the solar corona, evocatively referred to as “touching the Sun”. There would, however, be a gap between achieving this goal and knowing that it happened. Now the space agency has confirmed the Parker Solar Probe entered the corona in April, collecting data that will be studied for many years, eventually perhaps settling the longstanding question of why the corona is hotter than the Sun's surface.
Announced in Physical Review Letters, the passage through the corona is a first for a human-made instrument, adding to Parker Solar Probe's already record-breaking mission.
Not being solid, the Sun lacks an unambiguous boundary as to where it stops and space begins. The corona is referred to as the Sun's atmosphere and represents the region in which solar material is constrained by gravity and magnetic fields. Beyond it, the solar wind takes over, sending material permanently outwards from the Sun, rather than cycling towards and away as occurs within the corona.
While astronomers were confident a dividing line exists between the corona and the solar wind, known as the Alfvén critical surface, they were unsure of its location, with estimates ranging from 6.9-13.8 million kilometers (4.3 to 8.6 million miles) from the surface of the Sun. Moreover, they do know the correct figure would only be an average, with the corona expanding and contracting like an erratically played concertina. Whether a particular orbit passes through it could depend on the timing of its flight.
On April 28, Parker was 13 million kilometers (8.1 million miles), or 18.8 solar radii, from the Sun's surface and encountered particles and magnetic conditions expected to be found within the corona. The event was not announced immediately because it took a while to process the data, and NASA scientists then wanted to thoroughly check their findings rather than make a premature announcement.
"Parker Solar Probe “touching the Sun” is a monumental moment for solar science and a truly remarkable feat," said NASA's Dr Thomas Zurbuchen in a statement. "Not only does this milestone provide us with deeper insights into our Sun's evolution and its impacts on our solar system, but everything we learn about our own star also teaches us more about stars in the rest of the universe.”
Parker's exploration of the corona started well before it reached it. Magnetic structures, known as switchbacks, zig zag through the solar wind. In 2019 Parker revealed these are much more common than previously recognized. Since then Parker has got close enough to reveal at least some switchbacks start at the solar surface and are unusually rich in helium.
The probe even passed through a “pseudostreamer” an immense structure radiating from the Sun's surface seen during total eclipses. Inside the pseudostreamer Parker found the tumultuous conditions of the corona quietened down, with slower moving particles and fewer switchbacks. The Parker team attribute this to magnetic fields strong enough to dominate particle movement.
Even on the original pass, Parker went in and out of the corona several times, reflecting the wavy nature of the Alfvén critical surface, and the shallowness of the pass.
However, last month Parker passed considerably closer to the surface of the Sun. It's likely that this pass involved a much longer passage through the corona, with accompanying more detailed data. Provided it can resist the searing heat, future passes will take Parker closer still, reaching almost half the April pass's distance. Combined with an overall trend for the corona to expand as activity moves towards the next solar maximum, we can expect much more in-depth explorations.
Icarus, your dream is fulfilled.
